Vacuum settings

Hello,

I need wo advice on vacuum settings.

I have a quad core X5355 @ 2.66GHz with 8 Go of memory

1Q) Why autovaccum does not work, I have set the value to on in
postgresql.conf but when the server start it's still off !!!!

2Q) Here are my settings for vacuum, could you help me to optimise those
settings, at the moment the vacuum analyse sent every night is taking
around 18 h to run, which slow down the server performance.

# - Cost-Based Vacuum Delay -

vacuum_cost_delay = 5 # 0-1000 milliseconds
vacuum_cost_page_hit = 1000 # 0-10000 credits
vacuum_cost_page_miss = 1000 # 0-10000 credits
vacuum_cost_page_dirty = 120 # 0-10000 credits
vacuum_cost_limit = 20 # 0-10000 credits

# - Background writer -

bgwriter_delay = 50 # 10-10000 milliseconds between
rounds
bgwriter_lru_percent = 1.0 # 0-100% of LRU buffers
scanned/round
bgwriter_lru_maxpages = 25 # 0-1000 buffers max written/round
bgwriter_all_percent = 0.333 # 0-100% of all buffers
scanned/round
bgwriter_all_maxpages = 50 # 0-1000 buffers max written/round

Thanks in advance for your helps

Regards

David